Yes, you can row in a boat, in a lake, in the middle of the city. Right here.
This park is located between Campo Grande and Alvalade and it the biggest green area in the city center. Just so you can picture it, it has the size of 11 football fields, with a 2,7km perimeter.
Very close to the Lisbon University campus, you can find lots of students there, chilling or working, in the park or in the cafeterias that serve the area.
It is also very pleasant for families to engage in lots of recreational activities. Here are some of the things you can do if you visit:
See the statues, have a coffee or a snack, play a game of Padel, have a picnic in the grass, rent a rowing boat with your SO, walk your dogs in the canine park, visit the Ames room in the middle of the garden, take your kids to the children park or even learn math at the 4 mathematical benches you can find there.